Ultrahigh molecular weight polyethylene (“UHMWPE”) is commonly used in making orthopaedic implants, such as artificial hip joints. In recent years, it has become increasingly apparent that tissue necrosis and osteolysis at the interface of the orthopaedic implant and the host bone are primary contributors to the long-term loosening failure of prosthetic joints. It is generally accepted by orthopaedic surgeons and biomaterials scientists that this tissue necrosis and osteolysis is due, at least in part, to the presence of microscopic particles of UHMWPE produced during the wear of the UHMWPE components. The reaction of the body to these particles includes inflammation and deterioration of the tissues, particularly the bone to which the orthopaedic implant is anchored. Eventually, the orthopaedic implant becomes painful and/or loose and must be revised and/or replaced.
In order to increase the useful life of orthopaedic implants having UHMWPE parts, several attempts have been made to increase the wear resistance of the UHMWPE, thereby decreasing the number of wear particles that can cause tissue necrosis and/or osteolysis. One method for increasing the wear resistance of UHMWPE utilizes exposure to high-energy radiation, such as gamma radiation, in an inert or reduced-pressure atmosphere to induce cross-linking between the polyethylene molecules. This cross-linking creates a three-dimensional network of polyethylene molecules within the polymer which renders it more resistant to wear, such as adhesive wear. However, the free radicals formed upon irradiation of UHMWPE can also participate in oxidation reactions which reduce the molecular weight of the polymer via chain scission, leading to degradation of physical properties, embrittlement, and a significant increase in wear rate. Moreover, the three-dimensional network produced by the cross-linking reaction can reduce the mechanical properties of the UHMWPE.
There are several processes that have been developed to effectively and efficiently reduce the number of free radicals present in irradiated UHMWPE, all of which have met with varying degrees of success (see, e.g. U.S. Pat. No. 5,414,049). Moreover, while the cross-linking of the UHMWPE and other known methods can increase the wear resistance of a medical implant or medial implant part comprising UHMWPE, such implants or implant parts can still produce microscopic wear particles of UHMWPE that can lead to the eventual failure of the medical implant or medical implant part.
A need exists for alternative orthopaedic implants comprising UHMWPE and methods for producing and using such implants. The invention provides such an implant and such methods. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.